Magnetoresistive random access memory element and fabrication method thereof

ABSTRACT

A magnetoresistive random access memory (MRAM) element includes a bottom electrode embedded in a first insulating layer; an annular reference layer in a first via hole of a second insulating layer on the first insulating layer, the annular reference layer being situated above the bottom electrode; a first gap fill material layer filling the first via hole; a barrier layer covering the annular reference layer, the second insulating layer and the first gap fill material layer; an annular free layer in a second via hole of a third insulating layer on the second insulating layer, the annular free layer being situated above the annular reference layer; and a top electrode stacked on the annular free layer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to the filed of non-volatile memory technology. More particularly, the present invention relates to a magnetoresistive random access memory (MRAM) element with improved magnetization efficiency, and method for fabricating the same.

2. Description of the Prior Art

As known in the art, magnetoresistive random access memory (MRAM) is a non-volatile computer memory technology. MRAM is six times faster than the current industry-standard memory, dynamic RAM (DRAM). It is almost as fast as static RAM (SRAM) and is much faster and suffers less degradation over time than flash memory. Unlike these technologies, MRAM uses magnetism instead of electrical charges to store data.

In general, the MRAM cells include a data layer and a reference layer. The data layer is composed of a magnetic material and during a write operation the magnetization of the data layer can be switched between two opposing states by an applied magnetic field and thus binary information can be stored. The reference layer usually is composed of a magnetic material in which the magnetization is pinned so that the magnetic field, which is applied to the data layer and in part penetrates the reference layer, is of insufficient strength to switch the magnetization in the reference layer.

MRAM is physically similar to DRAM in makeup, although often does not require a transistor for the write operation. However, the most basic MRAM cell suffers from the half-select problem, which limits cell sizes. To be worth putting into wide production, however, it is generally believed that MRAM will have to move to the 65 nm size of the most advanced memory devices, which will require the use of spin-torque-transfer (STT) technology.

Spin-torque-transfer (STT) or Spin Transfer Switching, uses spin-aligned (“polarized”) electrons to directly torque the domains. Specifically, if the electrons flowing into a layer have to change their spin, this will develop a torque that will be transferred to the nearby layer. This lowers the amount of current needed to write the cells, making it about the same as the read process.

However, the prior art MRAM has several drawbacks. For example, the gap fill process of the reference layer using physical vapor deposition (PVD) methods becomes problematic when the aspect ratio of the gap is greater than 2 for example. Besides, as the cell packing density increases, the interference between neighboring cells is not negligible. Therefore, there is a need in this industry to provide an improved method for fabricating the MRAM devices in order to avoid the aforementioned PVD gap fill problem, as well as an improved MRAM structure that is capable of eliminating interference or coupling between neighboring cells.

SUMMARY OF THE INVENTION

To address these and other objects and in view of its purposes, the present invention provides a magnetoresistive random access memory (MRAM) element comprising a bottom electrode embedded in a first insulating layer; an annular reference layer in a first via hole of a second insulating layer on the first insulating layer, the annular reference layer being situated above the bottom electrode; a first gap fill material layer filling the first via hole; a barrier layer covering the annular reference layer, the second insulating layer and the first gap fill material layer; an annular free layer in a second via hole of a third insulating layer on the second insulating layer, the annular free layer being situated above the annular reference layer; and a top electrode stacked on the annular free layer.

According to another aspect, the invention provides a method for fabricating a MRAM element, comprising: providing a substrate; depositing a first insulating layer on the substrate; forming a bottom electrode in the first insulating layer; depositing a second insulating layer on the first insulating layer and the bottom electrode; forming a first via hole in the second insulating layer; forming an annular reference layer in the first via hole; filling the first via hole with a first gap fill material layer; depositing a barrier layer on the second insulating layer, the annular reference layer and the first gap fill material layer; depositing a third insulating layer on the barrier layer; forming a second via hole in the third insulating layer; forming an annular free layer in the second via hole; filling the second via hole with a second gap fill material layer; and forming a top electrode on the annular free layer.

These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. In the drawings:

FIGS. 1-8 are schematic, cross-sectional diagrams illustrating a method for fabricating MRAM element in accordance with one preferred embodiment of this invention;

FIGS. 9-13 are schematic, cross-sectional diagrams illustrating a method for fabricating MRAM element in accordance with another preferred embodiment of this invention; and

FIGS. 14-18 are schematic diagrams illustrating a method for fabricating MRAM element in accordance with still another preferred embodiment of this invention.

It should be noted that all the Figures are diagrammatic. Relative dimensions and proportions of parts of the drawings have been shown exaggerated or reduced in size, for the sake of clarity and convenience in the drawings. The same reference signs are generally used to refer to corresponding or similar features in modified and different embodiments.

DETAILED DESCRIPTION

In the following description, numerous specific details are given to provide a thorough understanding of the invention. However, it will be apparent to one skilled in the art that the invention may be practiced without these specific details. In order to avoid obscuring the present invention, some well-known system configurations and process steps are not disclosed in detail.

Likewise, the drawings showing embodiments of the apparatus are semi-diagrammatic and not to scale and, particularly, some of the dimensions are for the clarity of presentation and are shown exaggerated in the figures. Also, in which multiple embodiments are disclosed and described having some features in common, for clarity and ease of illustration and description thereof, like or similar features will ordinarily be described with like reference numerals.

The term “horizontal” as used herein is defined as a plane parallel to the conventional major plane or surface of the semiconductor substrate, regardless of its orientation. The term “vertical” refers to a direction perpendicular to the horizontal as just defined. Terms, such as “on”, “above”, “below”, “bottom”, “top”, “side” (as in “sidewall”), “higher”, “lower”, “over”, and “under”, are defined with respect to the horizontal plane

FIGS. 1-8 are schematic, cross-sectional diagrams illustrating a method for fabricating a magnetoresistive random access memory (MRAM) element in accordance with one preferred embodiment of this invention.

As shown in FIG. 1, a substrate 100 is provided. The substrate 100 may be a semiconductor substrate including but not limited to silicon substrate, silicon substrate with an epitaxial layer, SiGe substrate, silicon-on-insulator (SOI) substrate, gallium arsenide (GaAs) substrate, gallium arsenide-phosphide (GaAsP) substrate, indium phosphide (InP) substrate, gallium aluminum arsenic (GaAlAs) substrate, or indium gallium phosphide (InGaP) substrate. A semiconductor switching device 10 such as a field effect transistor is fabricated on a main surface of the substrate 100.

An insulating layer 14 is deposited on a main surface of the substrate 100 and covers the semiconductor switching device 10. A bottom electrode 16 is inlaid in the insulating layer 14 and is electrically connected to a terminal 12 such as a source or a drain of the semiconductor switching device 10. The bottom electrode 16 may be composed of metals such as tungsten, titanium, titanium nitride, tantalum or tantalum nitride, copper, gold, platinum, alloys thereof, or silicides thereof. It is to be understood that in other cases the bottom electrode 16 may be electrically connected to other types of control components. An insulating layer 18 overlies the insulating layer 14 and the bottom electrode 16. For example, the insulating layer 18 may be a silicon oxide film that can be formed by conventional chemical vapor deposition (CVD) methods.

As shown in FIG. 2, after the deposition of the insulating layer 18, a via etching process such as a plasma dry etching process is carried out to form a via hole 18 a in the insulating layer 18. Via hole 18 a exposes a top surface of the bottom electrode 16. According to one embodiment, via hole 18 a has substantially vertical sidewall with respect to the main surface of the substrate 100. Via hole 18 a may have a circular shape, an oval shape, a rectangular shape or a polygonal shape when viewed from the top.

As shown in FIG. 3, after the formation of via hole 18 a in the insulating layer 18, a magnetic material layer 20 is conformally deposited on the insulating layer 18 and on the interior surface of via hole 18 a by physical vapor deposition (PVD) or atomic layer deposition (ALD) methods. The magnetic material layer 20 is a thin film with uniform thickness and the magnetic material layer 20 does not fill up the via hole 18 a. According to one embodiment, the magnetic material layer 20 is a pinned magnetic layer, which may be composed of a material including but not limited to NiFe, NiFeCo, CoFe, CoFeB, Fe, Co, Ni, alloys or compounds thereof, and/or other magnetic materials.

Subsequently, as shown in FIG. 4, an anisotropic dry etching process is carried out to etch the magnetic material layer 20 thereby forming an annular sidewall spacer 20 a extending vertically along the sidewall of the via hole 18 a. The magnetic material layer 20 outside the via hole 18 a is removed to reveal the top surface of the insulating layer 18. After the etching back of the magnetic material layer 20, an insulating layer 21 such as silicon oxide is deposited over the substrate 100. The insulating layer 21 fills up the remaining space in the via hole 18 a and covers the sidewall spacer 20 a and the top surface of the insulating layer 18. Thereafter, a chemical mechanical polishing (CMP) is performed to remove the insulating layer 21 outside the via hole 18 a, a top portion of the sidewall spacer 20 a and a top portion of the insulating layer 18. The remaining sidewall spacer 20 a acts as a reference layer of the MRAM stack.

As shown in FIG. 5, a barrier layer 22 such as MgO or Al₂O₃ is deposited on the insulating layer 18 and the insulating layer 21. An insulating layer 24 such as silicon oxide is then deposited on the barrier layer 22. After the deposition of the insulating layer 24, a via etching process such as a plasma dry etching process is carried out to form a via hole 24 a in the insulating layer 24. The via hole 24 a exposes a portion of the barrier layer 22 and is situated directly above the sidewall spacer 20 a. According to the embodiment, the via hole 24 a has substantially vertical sidewall with respect to the main surface of the substrate 100. The via hole 24 a may have a circular shape, an oval shape, a rectangular shape or a polygonal shape when viewed from the top.

As shown in FIG. 6, after the formation of the via hole 24 a in the insulating layer 24, a magnetic material layer 26 is conformally deposited on the insulating layer 24 and on the interior surface of the via hole 24 a by physical vapor deposition (PVD) or atomic layer deposition (ALD) methods. The magnetic material layer 26 is a thin film with uniform thickness and the magnetic material layer 26 does not fill up the via hole 24 a. According to the embodiment, the magnetic material layer 26 has a thickness that is greater than that of the magnetic material layer 20. According to the embodiment, the magnetic material layer 26 may be composed of a material including but not limited to NiFe, NiFeCo, CoFe, CoFeB, Fe, Co, Ni, alloys or compounds thereof, and/or other magnetic materials.

As shown in FIG. 7, an anisotropic dry etching process is carried out to etch the magnetic material layer 26 thereby forming an annular sidewall spacer 26 a extending vertically along the sidewall of the via hole 24 a. The magnetic material layer 26 outside the via hole 24 a is removed to reveal the top surface of the insulating layer 24. After the etching back of the magnetic material layer 26, an insulating layer 27 such as silicon oxide is deposited over the substrate 100. The insulating layer 27 fills up the remaining space in the via hole 24 a and covers the sidewall spacer 26 a and the top surface of the insulating layer 24. Thereafter, CMP is performed to remove the insulating layer 27 outside the via hole 24 a, a top portion of the sidewall spacer 26 a and a top portion of the insulating layer 24. The remaining sidewall spacer 26 a acts as a free layer (or data layer) of the MRAM stack.

As shown in FIG. 8, after the formation of the annular free layer of the MRAM stack, a top electrode 28 is formed on the sidewall spacer 26 a. A bit line (not shown) may be electrically connected to the top electrode 28. The top electrode 28 may be composed of metals such as tungsten, titanium, titanium nitride, tantalum or tantalum nitride, copper, gold, platinum, alloys thereof, or silicides thereof. It is to be understood that in other cases the bottom electrode 16 may be electrically connected to other types of control components. Since the reference layer 20 b and the free layer 26 a are both annular, the close magnetic loop provides high magnetization efficiency and the interference between neighboring cells can be avoided.

FIGS. 9-13 are schematic, cross-sectional diagrams illustrating a method for fabricating MRAM element in accordance with another preferred embodiment of this invention, wherein like numeral numbers designate like elements, layers or regions.

As shown in FIG. 9, likewise, a substrate 100 is provided. The substrate 100 may be a semiconductor substrate including but not limited to silicon substrate, silicon substrate with an epitaxial layer, SiGe substrate, silicon-on-insulator (SOI) substrate, gallium arsenide (GaAs) substrate, gallium arsenide-phosphide (GaAsP) substrate, indium phosphide (InP) substrate, gallium aluminum arsenic (GaAlAs) substrate, or indium gallium phosphide (InGaP) substrate. A semiconductor switching device 10 a and a semiconductor switching device 10 b such as field effect transistors are fabricated on the main surface of the substrate 100.

An insulating layer 14 is deposited on the main surface of the substrate 100 and covers the semiconductor switching devices 10 a and 10 b. A bottom electrode 16 a and a bottom electrode 16 b are inlaid in the insulating layer 14 and may be electrically connected to a terminal 12 a and a terminal 12 b respectively of the semiconductor switching devices 10 a and 10 b. The bottom electrodes 16 a and 16 b may be composed of metals such as tungsten, titanium, titanium nitride, tantalum or tantalum nitride, copper, gold, platinum, alloys thereof, or silicides thereof. It is to be understood that in other cases the bottom electrodes 16 a and 16 b may be electrically connected to other types of control components. An insulating layer 18 overlies the insulating layer 14 and the bottom electrodes 16 a and 16 b. For example, the insulating layer 18 may be a silicon oxide film that can be formed by conventional chemical vapor deposition (CVD) methods.

After the deposition of the insulating layer 18, a via etching process such as a plasma dry etching process is carried out to form a via hole 18 a in the insulating layer 18. The via hole 18 a exposes a top surface of the bottom electrodes 16 a and 16 b and a portion of the insulating layer 18 between the bottom electrodes 16 a and 16 b. According to the embodiment, the via hole 18 a has substantially vertical sidewall with respect to the main surface of the substrate 100. The via hole 18 a may have a circular shape, an oval shape, a rectangular shape or a polygonal shape when viewed from the top.

After the formation of the via hole 18 a in the insulating layer 18, a magnetic material layer 20 is conformally deposited on the insulating layer 18 and on the interior surface of the via hole 18 a by PVD or ALD methods. The magnetic material layer 20 is a thin film with uniform thickness and the magnetic material layer 20 does not fill up the via hole 18 a. According to the embodiment, the magnetic material layer 20 is a pinned magnetic layer, which may be composed of a material including but not limited to NiFe, NiFeCo, CoFe, CoFeB, Fe, Co, Ni, alloys or compounds thereof, and/or other magnetic materials.

Subsequently, as shown in FIG. 10, an anisotropic dry etching process is carried out to etch the magnetic material layer 20 thereby forming a sidewall spacer 20 a and a sidewall spacer 20 b extending vertically along two opposite sidewalls of the via hole 18 a. The sidewall spacer 20 a is situated directly above the bottom electrode 16 a and the sidewall spacer 20 b is situated directly above the bottom electrode 16 b. The sidewall spacer 20 a is separated from the sidewall spacer 20 b. The magnetic material layer 20 outside the via hole 18 a is removed to reveal the top surface of the insulating layer 18. The sidewall spacers 20 a and 20 b act as a reference layer.

As shown in FIG. 11, an insulating layer 21 such as silicon oxide is then deposited over the substrate 100. The insulating layer 21 fills up the remaining space in the via hole 18 a and covers the sidewall spacers 20 a, 20 b and the top surface of the insulating layer 18. Thereafter, the insulating layer 21 outside the via hole 18 a, a top portion of the sidewall spacer 20 a and a top portion of the insulating layer 18 are removed by CMP.

As shown in FIG. 12, after CMP, a barrier layer 22 such as MgO or Al₂O₃ is deposited on the insulating layer 18 and the insulating layer 21.

As shown in FIG. 13, an insulating layer 24 such as silicon oxide is then deposited on the barrier layer 22. A free layer 26 a and a free layer 26 b are inlaid in the insulating layer 24. The free layer 26 a is situated directly above the sidewall spacer 20 a and the free layer 26 b is situated directly above the sidewall spacer 20 b. For example, a magnetic material layer is first deposited on the barrier layer 22 in a blanket manner. The magnetic material layer is then patterned to form the free layers 26 a and 26 b. Thereafter, the insulating layer 24 is deposited to cover the free layers 26 a and 26 b and the barrier layer 22. The excess insulating layer 24 may be removed by CMP. The thin sidewall spacers 20 a and 20 b that act as a reference layer provide high magnetization efficiency.

FIGS. 14-18 are schematic diagrams illustrating a method for fabricating MRAM element in accordance with still another preferred embodiment of this invention, wherein like numeral numbers designate like elements, layers or regions.

As shown in FIG. 14, likewise, an insulating layer 14 is provided on a substrate (not shown). A bottom electrode 16 a and a bottom electrode 16 b are inlaid in the insulating layer 14. The bottom electrodes 16 a and 16 b may be composed of metals such as tungsten, titanium, titanium nitride, tantalum or tantalum nitride, copper, gold, platinum, alloys thereof, or silicides thereof. An insulating layer 18 overlies the insulating layer 14 and the bottom electrodes 16 a and 16 b. For example, the insulating layer 18 may be a silicon oxide film that can be formed by conventional CVD methods. After the deposition of the insulating layer 18, a via etching process is carried out to form a via hole 18 a in the insulating layer 18. The via hole 18 a exposes a top surface of the bottom electrodes 16 a and 16 b and a portion of the insulating layer 18 between the bottom electrodes 16 a and 16 b. According to the embodiment, the via hole 18 a has substantially vertical sidewall with respect to the main surface of the substrate.

A magnetic material layer 20 is conformally deposited on the insulating layer 18 and on the interior surface of the via hole 18 a by PVD or ALD methods. The magnetic material layer 20 is a thin film with uniform thickness and the magnetic material layer 20 does not fill up the via hole 18 a. According to the embodiment, the magnetic material layer 20 is a pinned magnetic layer, which may be composed of a material including but not limited to NiFe, NiFeCo, CoFe, CoFeB, Fe, Co, Ni, alloys or compounds thereof, and/or other magnetic materials.

As shown in FIG. 15, an anisotropic dry etching process is then carried out to etch the magnetic material layer 20 thereby forming a sidewall spacer 20 a and a sidewall spacer 20 b extending vertically along two opposite sidewalls of the via hole 18 a. The sidewall spacer 20 a is situated directly above the bottom electrode 16 a and the sidewall spacer 20 b is situated directly above the bottom electrode 16 b. The sidewall spacer 20 a is separated from the sidewall spacer 20 b. The magnetic material layer 20 outside the via hole 18 a is removed to reveal the top surface of the insulating layer 18. Subsequently, a patterned sacrificial layer 32 such as silicon oxide, silicon nitride or photoresist is formed on the insulating layer 18 and the patterned sacrificial layer 32 covers a portion of the sidewall spacers 20 a and 20 b.

As shown in FIG. 16, a wet etching process is then carried out to etch the sidewall spacers 20 a and 20 b that are not covered with the patterned sacrificial layer 32. Thereafter, a portion of the sidewall spacer 20 a under the patterned sacrificial layer 32 and a portion of the sidewall spacer 20 b under the patterned sacrificial layer 32 are etched away to form a pillar 120 a and a pillar 120 b respectively on the bottom electrodes 16 a and 16 b. The pillar 120 a and a pillar 120 b act as a reference layer.

As shown in the FIG. 17, after the formation of the pillars 120 a and 120 b, the patterned sacrificial layer 32 is removed to reveal the pillars 120 a and 120 b. Removal of the patterned sacrificial layer 32 may be implemented by methods known in the art, for example, wet etching, dry etching or plasma ashing.

As shown in FIG. 18, an insulating layer 21 such as silicon oxide is then deposited. The insulating layer 21 fills up the remaining space in the via hole 18 a and covers the pillars 120 a and 120 b and the top surface of the insulating layer 18. Thereafter, at least the insulating layer 21 outside the via hole 18 a is removed by CMP. A barrier layer 22 such as MgO or Al₂O₃ is then deposited on the insulating layer 18 and the insulating layer 21. An insulating layer 24 such as silicon oxide is then deposited on the barrier layer 22. A free layer 26 a and a free layer 26 b are inlaid in the insulating layer 24. The free layer 26 a is situated directly above the sidewall spacer 20 a and the free layer 26 b is situated directly above the sidewall spacer 20 b. For example, a magnetic material layer is first deposited on the barrier layer 22 in a blanket manner. The magnetic material layer is then patterned to form the free layers 26 a and 26 b. Thereafter, the insulating layer 24 is deposited to cover the free layers 26 a and 26 b and the barrier layer 22. The excess insulating layer 24 may be removed by CMP. Top electrodes 28 a and 28 b are then formed on the free layers 26 a and 26 b respectively. The pillars 120 a and 120 b that act as the reference layer provide high polarization efficiency and magnetization efficiency.

Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. 

1. A magnetoresistive random access memory (MRAM) element, comprising: a bottom electrode embedded in a first insulating layer; an annular reference layer in a first via hole of a second insulating layer on the first insulating layer, the annular reference layer being situated above the bottom electrode; a first gap fill material layer filling the first via hole; a barrier layer covering the annular reference layer, the second insulating layer and the first gap fill material layer; an annular free layer in a second via hole of a third insulating layer on the second insulating layer, the annular free layer being situated above the annular reference layer; and a top electrode stacked on the annular free layer.
 2. The MRAM element according to claim 1 further comprising a second gap fill material layer filling into the second via hole of the third insulating layer.
 3. The MRAM element according to claim 1 wherein the annular reference layer is formed on sidewall of the first via hole.
 4. The MRAM element according to claim 1 wherein the annular free layer is formed on sidewall of the second via hole.
 5. The MRAM element according to claim 1 wherein the first insulating layer is deposited on a substrate.
 6. The MRAM element according to claim 5 wherein at least one semiconductor switching device is fabricated on the substrate.
 7. The MRAM element according to claim 6 wherein the bottom electrode is electrically connected to a terminal of the semiconductor switching device.
 8. The MRAM element according to claim 1 wherein the annular reference layer is made of pinned magnetic material.
 9. The MRAM element according to claim 1 wherein the annular reference layer comprises NiFe, NiFeCo, CoFe, CoFeB, Fe, Co, Ni, alloys or compounds thereof.
 10. The MRAM element according to claim 1 wherein the annular free layer comprises NiFe, NiFeCo, CoFe, CoFeB, Fe, Co, Ni, alloys or compounds thereof.
 11. The MRAM element according to claim 1 wherein the barrier layer comprises MgO or Al₂O₃.
 12. A method for fabricating a magnetoresistive random access memory (MRAM) element, comprising: providing a substrate; depositing a first insulating layer on the substrate; forming a bottom electrode in the first insulating layer; depositing a second insulating layer on the first insulating layer and the bottom electrode; forming a first via hole in the second insulating layer; forming an annular reference layer in the first via hole; filling the first via hole with a first gap fill material layer; depositing a barrier layer on the second insulating layer, the annular reference layer and the first gap fill material layer; depositing a third insulating layer on the barrier layer; forming a second via hole in the third insulating layer; forming an annular free layer in the second via hole; filling the second via hole with a second gap fill material layer; and forming a top electrode on the annular free layer.
 13. The method for fabricating the MRAM element according to claim 12 wherein forming the annular reference layer in the first via hole comprises the steps of: conformally depositing a magnetic material layer on the second insulating layer and on interior surface of the via hole; and anisotropically etching the magnetic material layer thereby forming the annular sidewall spacer extending vertically along sidewall of the via hole.
 14. The method for fabricating the MRAM element according to claim 12 wherein after filling the first via hole with the first gap fill material layer, a chemical mechanical polishing (CMP) is carried out.
 15. The method for fabricating the MRAM element according to claim 12 wherein forming the annular free layer in the second via hole comprises the steps of: conformally depositing a magnetic material layer on the third insulating layer and on interior surface of the via hole; and anisotropically etching the magnetic material layer thereby forming the annular sidewall spacer extending vertically along sidewall of the via hole.
 16. The MRAM element according to claim 12 wherein at least one semiconductor switching device is fabricated on the substrate.
 17. The MRAM element according to claim 16 wherein the bottom electrode is electrically connected to a terminal of the semiconductor switching device.
 18. The MRAM element according to claim 12 wherein the annular reference layer is made of pinned magnetic material.
 19. The MRAM element according to claim 12 wherein the annular reference layer comprises NiFe, NiFeCo, CoFe, CoFeB, Fe, Co, Ni, alloys or compounds thereof.
 20. The MRAM element according to claim 12 wherein the annular free layer comprises NiFe, NiFeCo, CoFe, CoFeB, Fe, Co, Ni, alloys or compounds thereof.
 21. The MRAM element according to claim 12 wherein the barrier layer comprises MgO or Al₂O₃. 